This invention relates generally to solar energy concentrators, and, more particularly to an improved solar energy concentrator system which incorporates therein a totally enclosed novel solar concentrator capable of operating in a highly reliable and efficient manner.
With the increased cost encountered for the production of energy, as a result of, for example the depletion of fossil fuels as a source of energy, more and more effort has been directed toward utilizing the energy of the sun as a means of producing an alternate source of usable energy. A recently developed technique utilizes photovoltaic cells in a solar energy system for directly converting sunlight into electricity. Such a system generally comprises a field of parabolic trough solar concentrators that direct sunlight onto receiver tubes mounted with photovoltaic cells that run the length of the concentrators. The photovoltaic cell, which has the property of converting sunlight directly into electricity, feeds the electricity through a conversion unit into a local utility grid from which power can be drawn.
Unfortunately, because of the relative movement between the earth and the sun, the angles of the incident rays from the sun upon the concentrators continuously vary. To accommodate this continuous change, various movable arrangements of mirrors have been coupled with various control means to cause these mirrors to automatically track the sun as the earth rotates, the energy then being concentrated at a fixed receiver. There are disadvantages in these types of arrangements for large scale production of heat, and these include the high cost of mechanically supporting the structures, many of which are complex, for effective operation.
An even more pressing cause for concern in systems utilizing photovoltaic cell arrays is a phenomena called cell shadowing. This phenomena occurs at the ends of concentrator modules. Basically, these end effects (cell shadowing) are due to the systems requirement to combine a plurality of concentrator modules into a concentrator array. Inherent in such systems are voids located between neighboring concentrator modules of an array. These voids effectively shadow the photovoltaic cells that are dependent upon the irradiated solar flux of the non-existent reflector surface.
Stated more succinctly, reflectors which have been built to date are designed in a rectanglar configuration with straight sides normal to the focal axis of the reflector. Accordingly, the void caused by neighboring reflectors of adjacent concentrator modules is a right circular parabolic segment which would have focused the incident energy at one point on the photovoltaic cell if the reflector surface was present. The lack of reflected energy at that point results in what is known as a shaded area and creates the cell shadowing described above.
In the case, for example, where a concentrator module incorporates therein thirty-six photovoltaic cells, if only one such cell were shaded 50% of the time, the current output of the entire thirty-six series connected string of cells would be reduced approximately 40%. In effect, the shadowing current limits the solar conversion capability of the cell, after which the cell current limits the string of cells which are connected in series. If this string represents a significant part of the photovoltaic receiver, it is readily apparent that there would be a serious power loss.
To overcome the possible shortcoming due to shading of cells, shunting diodes have been installed for each cell so that as current drops to a predetermined level, the shunt becomes active and drops the shaded cells out of the series circuit. There would, however, still be a power loss of about 3% in a thirty-six string receiver. The converse is also true, as the shading is removed the current increases until it reaches the shunting value beyond which it automatically comes on line.
Although solar energy systems utilizing shunting diodes are more effective than those that do not, their use within the system is not only extremely costly but also still produces systems which contain power losses of about 3%. Furthermore, with the addition of the shunting diodes within the system a new source of malfunction can occur within the system, and, if a diode fails, a drastic reduction in energy output can result. Such a problem may require a complete shutdown of the system in order to find and replace the failing diode.
An additional drawback associated with currently available solar energy systems is the inability to keep the system clean so as to utilize maximum solar energy input. Furthermore, the concentrator modules are cumbersome and therefore must be shut down under high wind conditions in which the structure is adversely affected by the surrounding environmental conditions. In instances in which the system is to be operational full time substantial expense is involved in order to increase the structural rigidity of the system in order to maintain alignment of the concentrators with the sun even under extreme environmental conditions.
It is therefore clearly evident from the above description that although solar energy concentrator systems are potentially a highly desirable means for providing usable energy, there are still many drawbacks associated with such systems. It would therefore be beneficial to provide a solar energy concentrator system which is capable of substantially eliminating the problems encountered with past solar energy systems.